1. Technical Field
The disclosure relates to a multicast communication method, accompanied with the relay node and wireless network system design.
2. Background
Wireless sensor network will be broadly applied in our daily life along with the development of wireless sensor network technologies. Taking the ZigBee specification based on the IEEE 802.15.4 standard as an example, a multicast method is specified in the ZigBee specification document, wherein information is broadcasted to nodes belonging to a specific group. The multicast method specified in the ZigBee standard has a non-member mode and a member mode. In the non-member mode, packets are transmitted in a one-to-one (i.e., one node to one node) manner with a unicast, while in the member mode, packets are broadcasted. When a multicast communication starts, an initial node appoints a specific group as the target and issues a multicast packet to the specific group, wherein the initial node determines whether it belongs to the specific group before it issues the multicast packet. If the initial node does not belong to the specific group, it transmits the multicast packet to the next node in non-member mode. The multicast packet is transmitted in non-member mode until it reaches a member in the specific group, and then the multicast packet will be transmitted in member mode thereafter. To be specific, in the multicast method specified in the ZigBee specification document, a MaxNonMemberRadius field and a NonMemberRadius field are defined in each packet. The MaxNonMemberRadius field defines a radius (i.e., number of hops) which the minimum distance between a node in the specific group and any other node in the specific group should not exceed. For example, when the radius which should not be exceeded by the minimum distance is set to 2 hops, two nodes in the same group should be able to communicate within their communication ranges or via at most two nodes which do not belong to the group. The NonMemberRadius field records the number of nodes which do not belong to the specific group and are involved in the transmission of a multicast packet. Thus, when a node in the wireless sensor network receives the multicast packet, the node determines whether it has received the multicast packet before. The node discards the multicast packet if it has already received the multicast packet. If the node has not received the multicast packet before, the node checks whether it belongs to the specific group. If the node belongs to the specific group, it sets the value in the NonMemberRadius field to the value in the MaxNonMemberRadius field and broadcasts the received multicast packet. If the node does not belong to the specific group, it inspects the value in the NonMemberRadius field, wherein if the value in the NonMemberRadius field is 0, the relaying of the multicast packet is terminated; if the value in the NonMemberRadius field is 7, the relay number of the multicast packet is not limited and it is continued to relay the multicast packet; and if the value in the NonMemberRadius is between 1 and 6, the value in the NonMemberRadius field is decreased by 1 and the received multicast packet is relayed.
However, in the multicast method described above, even though a group is specified in the ZigBee standard to reduce the possibility of repeated packet and packet collision, in a network of higher node density, the power consumption will still be very high, and the power of those devices having lower power will be consumed quickly so that the operation time and lifespan of these devices will be shortened. Therefore, a multicast communication method which can reduce and balance the power consumption of all the nodes (for example, wireless sensors) and at the same time, can effectively broadcast information packets to member nodes in a specific group is desired.